A capacitance type acceleration sensor which can perform self-diagnosis is disclosed in JP-A-2000-81449. This sensor will be described hereunder with reference to FIGS. 9, 10 and 11.
In plan view of FIG. 9, a beam arrangement structure 101 is formed on a substrate 100. The beam arrangement structure 101 has movable electrodes 101a displaceable in an acceleration detecting direction corresponding to the Y-direction parallel to the surface of the substrate 100. Fixed electrodes 102 and 103 are disposed on the substrate 100 so as to confront each other and so that each of the movable electrodes 101a of the beam arrangement structure 101 is sandwiched therebetween in the Y direction. When an acceleration is applied and thus the movable electrodes 101a of the beam arrangement structure 101 are displaced, the capacitance between each movable electrode 101a and each fixed electrode 102 and the capacitance between each movable electrode 101a and each fixed electrode 103 are differentially varied.
As shown in FIG. 11, this sensor has a period (phase φ1, φ2) for detecting capacitance variation under self-diagnosis and a period (phase φ3) for displacing the movable electrodes 101a. In the period (phase φ1, φ2) during which a periodically-varying signal is applied between each movable electrode 101a and each fixed electrode 102, 103 to detect the capacitance variation therebetween, the voltage corresponding to the variation of the differential capacitance comprising the movable electrode 101a and the fixed electrode 102, 103 is output by a C-V conversion circuit 200 of FIG. 10. In the period (the phase φ3 of FIG. 11) during which the movable electrodes 101a are displaced, a voltage applied to the non-inverting input terminal of an operational amplifier 201 of the C-V conversion circuit 200 of FIG. 10 is switched from Vcc/2 to a self-diagnosis voltage V1 and a pseudo acceleration is applied to the movable electrodes 101a. 
The operation under the self-diagnosis will be described in detail with reference to FIG. 10.
Under the self-diagnosis, a self-diagnosis signal TEST is input to a control circuit 300. The control circuit 300 outputs a signal indicated in FIG. 11 to set a switch signal S3 to high level and also set a switch signal S3 (bar) to low level in a third period φ3. As a result, in the third period φ3, a switch 400b is closed and a switch 400a is opened, so that the self-diagnosis voltage V1 is applied to the non-inverting input terminal of the operational amplifier 201. At this time, since a switch 202 is closed, the operational amplifier 201 serves as a voltage follower, so that a potential difference V1 occurs between the movable electrode 101a and the fixed electrode 102 and a potential difference (Vcc−V1) occurs between the movable electrode 101a and the fixed electrode 103. Therefore, electrostatic forces occur between the movable electrode 101a and the fixed electrode 102 and between the movable electrode 101a and the fixed electrode 103 so as to be reciprocal to each other, and the movable electrodes 101a are forced to be displaced by the differential force of the electrostatic forces.
This electrostatic force would occur at a sufficiently higher frequency than the resonance frequency in the detection direction of the movable electrodes 101a if the frequencies of carrier signals P1, P2 of FIG. 11 are set to sufficiently higher frequencies than the resonance frequency (for example, twice or more frequency), and thus the movable electrodes 101a are set to such a state that a DC-like acceleration apparently occurs in the movable electrodes 101a. At this time, the self-diagnosis can be performed by detecting the DC-like displacement of the movable electrodes 101a as a capacitance variation.
However, the approach for performing self-diagnosis in the sensor of FIG. 9 in which the Y-direction horizontal to the substrate surface is set to the detection direction is not sufficient for a sensor in which the detection direction is a Z-direction vertical to the substrate surface because the support substrate cannot be set to a desired potential to displace the movable electrodes.